(1) Field of the Invention
The present invention relates to a baked color pencil lead which can draw lines showing a vivid color and having excellent light fastness and weather resistance and which holds excellent physical strength, that is, flexural strength, tensile strength and impact strength, and a method of manufacturing the same.
(2) Description of the Related Art
Conventional baked color pencil leads have been manufactured by adding a filler such as boron nitride and according to necessity, a heat-resistant pigment and a reaction accelerator to at least one kind of clay as a binder, kneading the resulting blend composition, molding this kneaded matter by extrusion and then subjecting it to a heat treatment to prepare a porous baked lead, and filling the pores of this lead with an ink comprising a dye. In this case, required to leads are important characteristics such as strong mechanical strength, a good coloring property, a high density of drawn lines, and excellent light fastness and weather resistance.
However, conventional baked color pencil leads are not sufficient in high mechanical strength, a density and a coloring property, light fastness and weather resistance. Accordingly, in order to enhance the density and the coloring property, the amount of an ink to be filled into the leads has to be increased by raising the porosity while maintaining the high mechanical strengths.
Proposed as a method of increasing the porosity of a baked color pencil lead are, for example, a method in which a resin which is a forming binder is added to a filler and a binder, and this resin is sublimated or burned in an oxidizing atmosphere during baking to form pores (refer to Japanese Patent Publication Nos. Sho 64-4555 and Sho 51-41376), a method in which a granular carbon material is used as a pore-forming material, and this is burned in an oxidizing atmosphere to form pores (refer to Japanese Patent Application Laid-Open No. Sho 61-275370), and a method in which a fibrous material is used as a pore-forming material, and this is burned in an oxidizing atmosphere, or sublimated or melted by heating to high temperatures to form pores (refer to Japanese Patent Application Laid-Open No. Hei 5-302054).
However, a conventional baked lead using clay as a binder has a weak bonding force between a filler such as boron nitride and the binder, and the clay itself has low strength. Accordingly, the existing situation is that the resulting baked color pencil lead has not attained practical strength. Further, the clay contains impurities, so that the baked leads are usually colored and have the problem that an adverse effect is exerted on the coloring property of drawn lines. In particular, it causes a somber color of light-colored drawn lines.
Accordingly, proposed are a method using aluminum nitride as a white porous material having high strength (refer to Japanese Patent Application Laid-Open No. Sho 59-117570) and a method using silicon nitride (refer to Japanese Patent Application Laid-Open No. Sho 59-117569).
However, in the aforementioned former method using aluminum or an aluminum compound, it is difficult to obtain fine powder having a particle diameter of 2 .mu.m or less suitable for the binder, and the larger particle diameter provides the problem that it damages a writing feeling.
The silicon nitride used in the latter method described above has high strength and is very useful as the binder. Known as a method of manufacturing porous leads using this silicon nitride as the binder are:
(1) a method in which silicon nitride powder is kneaded with a filler and the like, and the mixture is molded, followed by baking under pressurizing in a nitrogen atmosphere, and PA1 (2) a method in which silicon powder is kneaded with a filler and the like, and the mixture is molded, followed by baking in a nitrogen atmosphere or an ammonia gas atmosphere to nitride. PA1 kneading a blend composition comprising at least a filler and an organic forming binder, molding the composition by extrusion, and baking the molded composition in a non-oxidizing atmosphere, thereby to form a first baked lead containing a binder of carbon produced by carbonizing the above organic forming binder, PA1 heating the first baked lead in an oxidizing atmosphere to remove the binder of the carbon by oxidation, thereby to form a second baked lead comprising at least the filler, PA1 impregnating the pores of the second baked lead with a perhydropolysilazane-containing solution and then subjecting it to a heat treatment in an inert atmosphere such as a nitrogen atmosphere or in an ammonia gas atmosphere, thereby to form a third baked lead containing silicon nitride, and PA1 impregnating the pores of the third baked lead with an organic pigment solution and solidifying the organic pigment in the pores to allow the pigment to be contained in the baked lead as a colorant. PA1 kneading a blend composition comprising at least a filler and an organic forming binder, molding the composition by extrusion, and baking the molded composition in a non-oxidizing atmosphere, thereby to form a first baked lead containing a binder of carbon, PA1 heating the first baked lead in an oxidizing atmosphere to remove the binder of the carbon by oxidation, thereby to form a second baked lead, PA1 impregnating the pores of the second baked lead with a perhydropolysilazane-containing solution and then subjecting it to a heat treatment in an inert atmosphere such as a nitrogen atmosphere or in an ammonia gas atmosphere, thereby to form a third baked lead containing silicon nitride, and PA1 impregnating the pores of the third baked lead with an organic pigment solution prepared by dissolving the organic pigment in at least one selected from an organic solvent, an acid solution and an alkali solution, and solidifying the organic pigment in the pores by at least one means selected from solvent evaporation, poor solvent replacement method for the pigment including water deposition, and neutralization to allow the pigment to be contained in the baked lead as a colorant. PA1 (1) the second baked lead formed by heating the first baked lead in an oxidizing atmosphere to remove the binder of carbon by oxidation is a porous lead having many pores, and silicon nitride can be produced as the binder in the second baked lead by impregnating the second baked lead having many pores with a perhydropolysilazane-containing solution and subjecting it to a heat treatment in an inert atmosphere such as a nitrogen atmosphere or in an ammonia gas atmosphere, which results in enhancing the mechanical strength of the above baked lead; PA1 (2) since colorless, transparent silicon nitride is obtained by using the perhydropolysilazane-containing solution, the third baked lead becomes white by selecting a white filler and exerts no adverse effects on the hue of lines drawn with the baked color pencil lead; further, since the resulting silicon nitride is excellent in solvent resistance, acid resistance and an alkali resistance, it becomes possible to impregnate the pores of the above baked color pencil lead with a pigment solution prepared by dissolving the pigment in an organic solvent, an acid solution or alkali solution, and the baked color pencil lead having a high chroma and a vivid color can be obtained by using at least the pigment solidified in the pores by means of water deposition, neutralization or solvent evaporation as a colorant; PA1 (3) in the second baked lead formed by heating the first baked lead in an oxiding atmosphere to remove the binder of the carbon by oxidation, the porosity is large, and the pores are fine and uniformly dispersed, and the filler is densely oriented as well, so that the third baked lead can be highly strengthened while holding the pores to be filled with a pigment solution prepared by dissolving the pigment in an organic solvent, an acid solution or an alkali solution, which is necessary to obtain the high density of drawn lines, by producing a small amount of silicon nitride having high strength in the second baked lead; PA1 (4) the fine and uniform dispersion of the silicon nitride existing in the third baked lead as the binder sharpens the pore distribution and allows the solidified pigment to be uniformly dispersed in the third baked lead; in addition, the uniform abrasion of the densely oriented filler at the time of writing equalizes the density of drawn lines and makes the writing feeling equal to or higher than that of conventional baked color pencil leads using clay for the binders; PA1 (5) unlike leads obtained merely by impregnating conventional light-colored or white porous leads with a dye-containing ink, the baked color pencil lead of the present invention contains at least the pigment as the colorant, so that the baked color pencil lead capable of drawing lines having excellent light fastness and weather resistance can be obtained.
However, the method (1) described above has the problem that it is difficult to obtain a color pencil lead having high strength even by applying pressure because of difficulty in baking silicon nitride.
Further, in the method (2) described above, the silicon powder having a particle diameter of 1 .mu.m or less is susceptible to a spontaneous oxidation to become silicon oxide powder, and much attention has to be paid for preventing the oxidation. That is because when carbon coexists, silicon oxide is nitrided by baking at 1200.degree. C. to 1300.degree. C. in a nitrogen atmosphere and causes growth in the vapor phase to form silicon nitride whiskers. Further, the larger particle diameter of the silicon powder causes the problem that the writing feeling is impaired.
Furthermore, since there are available no pigments which have a vivid hue as colorants and a high heat resistant temperature and which are harmless to a human body and usable to baked color pencil leads, dyes which can be impregnated after baking are used as colorants in conventional baked color pencils. However, the dyes have the problem that the drawn lines are inferior in light fastness and weather resistance. Leads having large pores make it possible to cause the pores to be impregnated with a pigment-dispersed ink but have the problem that they can not be used for baked color pencil leads because of a reduction in the strength.